1. Field of the Invention
The invention relates to an illumination system including at least one light source such as an electroluminescent element, e.g. a light emitting diode (LED), and at least one optical element whose surface is structured by means of diffraction and/or refraction type optical microstructures. The invention further relates to an optical element whose surface is structured by means of diffraction and/or refraction type optical microstructures, and to a collimating lens with such an optical element or such an illumination system.
2. Description of Related Art
LEDs are well known in the art, are available for a large variety of wavelengths, emitting powers and other properties and are used for many appliances. A key problem related to using LED sources is the fact that light emitted by a LED can not be easily shaped or collimated. This is due to the fact that a LED is an extended light source with an active area of typically a few tens of micrometers up to a few millimeters, and, therefore, has an output light distribution that extends over a broad angular range. In order to collimate or otherwise shape the emitted light beam, an external optical system has to be arranged next to the LED.
Collimation by single sided diffractive lenses, such as Fresnel zone plates, is known in the art. Furthermore, WO 2004/044995 suggests to arrange a micro-optical element with diffraction and/or refraction type optical microstructures on one of its surfaces next to the light source to collimate and/or otherwise shape the initial, typically Lambertian light distribution. WO 2004/044995 also proposes to use a micro-optical element with a plurality of independent sections, wherein each section has an individual optical function. WO 2004/044995 mentions a plurality of different possible functions of the micro-optical element. However, WO 2004/044995 is silent as to the concrete design of the optical element necessary to achieve these functions.
U.S. Pat. No. 5,577,492 discloses a lens using classical refractive optics to collimate radiant energy, e.g. solar light or light of a LED. The lens has two different sections. An inner section is shaped to focus light by means of a convex or a Fresnel lens. A ring-shaped outer section comprises a saw-tooth-like profile with a plurality of macroscopic facets. These facets are shaped in such a way that a ray entering the lens in the outer section is subject to Total Internal Reflection (TIR) at one side of the facet. U.S. Pat. No. 5,577,492 proposes to use a lens with only a few facets if light of a LED is to be collimated. Consequently, no optical microstructures are involved.